In recent years, a digital copying machine has a memory and performs an image processing procedure for once storing a digital image read from a scanner on the memory and sending the stored digital image to a printer. Further, a black and white copying machine often uses binary uncompressed data at the point when the digital image is read from the scanner because only a small capacity is required. On the other hand, since a color image has levels of gray of 8 bit/pixel for each color of R, G and B, it is difficult to store the color image in a memory as uncompressed data. Therefore, in the case of the color image, there has been proposed a method of performing data compression such as JPEG to store the color image.
On the other hand, in order to further increase speed of a high-speed copy mode of the digital copying machine, control for causing a printer to start printing when a predetermined amount of digital images from a scanner are stored in a memory (hereinafter referred to as follow-up control) is performed.
In a variable-length coding method such as the above-described JPEG, a compression rate is high, however, since a coding amount per a fixed time is not decided, time until the predetermined amount of digital images are stored in the memory is unfixed. As a result, timing for starting printing is lost, and the follow-up control cannot be performed.